Supercritical fluid extraction (SFE) is a sample preparation technique used to extract analytes of interest from a sample, for example, environmental pollutants from a soil sample. Some type of sample preparation must be performed for a wide range of environmental, food, polymer, petroleum, pharmaceutical and other classes of samples due to the complex nature of the samples. Many of these samples are so complex that they cannot be directly analyzed by analytical techniques such as gas chromatography (GC) or liquid chromatography (LC). The complex samples must first go through a sample preparation step to perform a gross separation of the analytes of interest from the sample matrix in which they are contained, for example, the environmental pollutants from soil. After the sample preparation step, then just the analytes of interest are analyzed by the analytical technique such as gas chromatography (GC), liquid chromatography (LC), or supercritical fluid chromatography (SFC). Other analytical techniques could be used such as mass spectroscopy (MS) or nuclear magnetic resource (NMR).
The most popular sample preparation steps are Soxhlet extraction and liquid extraction. An alternative to these types of extraction techniques is supercritical fluid extraction (SFE). SFE offers a relatively rapid, simple and inexpensive technique to perform sample preparations. The basis of SFE is that a fluid, such as carbon dioxide, is held at a specific pressure, temperature and flowrate, which is above its critical temperature and pressure and thus is a supercritical fluid. The supercritical fluid is passed through the sample matrix containing the analytes of interest. This sample matrix is contained in an extraction vessel. The fluid diffuses into the pores of the matrix, solubilizes the analytes of interest, and then carries the analytes away from the matrix. The analytes are then collected by some device, so that the analytes can be analyzed by some further analytical technique, such as chromatography. The matrix (now without analytes) is left behind in the extraction vessel. Supercritical fluids have favorable diffusivities and viscosities providing for good mass transfer characteristics. Their solvent strength can be easily controlled by changing fluid pressure or temperature. These are but a few of the advantages of supercritical fluid extraction.
Typically, an SFE system is comprised of a pump which pumps the supercritical fluid to an extraction vessel where analytes are extracted from a sample matrix. The analytes are then transported to a collection device where the supercritical fluid is depressurized to ambient pressure and is vented. The analysis of the collected analytes can be either "off-line", that is, remote from the extraction and/or collection device, or "on-line", that is, fluidically connected to the extraction and/or collection device.
The primary supercritical fluid used in SFE is carbon dioxide due to its low pressure and temperature critical points (71 atm, 31.degree. C., respectively) and its ability to solubilize nonpolar or moderately polar analytes. When it is desired to extract a polar analyte, then it is well known in the art to employ a co-solvent with the carbon dioxide. These co-solvents are typically referred to as modifiers or entrainers and are typically a liquid organic solvent such as methanol, ethanol, propylene carbonate, acetone, tetrahydrofuran, fomic acid, etc. that are blended with the carbon dioxide in 1 to 50% by volume or mole percent to form a mixture that retains much of the diffusion characteristics of the pure carbon dioxide phase but that has a much higher polarity and thus is able to solubilize polar analytes and extract the polar analytes from the sample matrix.
The prior art of SFE is quite large (see, for instance, U.S. Pat. No. 4,500,432, "Supercritical Fluid Technology", ACS Symposium Series 488, Chapter 12, McNally et al., 1991, pp. 144-164, "A Model for Dynamic Extraction Using Supercritical Fluid", Bartle et al., Journal of Supercritical Fluids", 1990, 3, 143-149, "Supercritical Fluid Extraction & Chromatography", ACS Symposium Series 366, Chapter 3, Wright et al., 1988, pp. 44-62, U.S. Pat. No. 4,597,943, European Patent No. 0444299A1, European Patent No. 0384969A2, European Patent No. 0458125A2, European Patent No. 0438184A1, U.S. Pat. No. 5,031,448, U.S. Pat. No. 5,013,443, U.S. Pat. No. 5,087,360, U.S. Pat. No. 4,984,602).
It has been generally known in the prior art that SFE must be automated to allow the technology to grow (see M. L. Bruce, "Beyond the Hype", August/September 1991, Environmental Lab). Only with a reliable, rapid, multisample analysis will the technique of supercritical fluid extraction be exploited to full advantage. One approach toward SFE automation is shown in "Supercritical Fluid Technology", ACS Symposium Series 488, Chapter 12, McNally et al., 1991, pp. 144-184. In this method, a series of extraction vessels (up to 12) are plumbed into a common rotary valve. The extraction vessels are not moved. The rotary valve is rotated to fluidically connect one of the extraction vessels to the plumbing of the extraction system. This relatively complex valve system is difficult to clean due to the numerous and essentially redundant fluid lines and is prone to leakage due to the rotary valve and the many plumbing fittings.
The basic needs of SFE are well known but the present invention has advantages over the prior art in a number of important areas including: 1) the ability to automate the SFE process so that it runs unattended for preferably up to 44 samples (or more); commensurate with 2) the ability to individually program each extraction vessel or fraction of a vessel with a different pressure, temperature, percent modifier, supercritical fluid flowrate and analyte collection method; and 3) the incorporation of a new variable restrictor that allows the effective decompression of the supercritical fluid into the collection system. The variable restrictor is unique due to its low dead volume, ability to control carbon dioxide flows in the 0.3 to 7.0 ml/min. range, and ability to keep the depressurized carbon dioxide from freezing in the outlet of the value.